1. Field of the Invention
This invention relates to a phase differential type focus detector used in an optical equipment such as an automatic focusing type single lens reflex camera (hereinafter, abbreviated as AF-SLR camera), and especially relates to a focus detector having at least two photoelectric transfer element arrays.
2. Description of the Prior Art
Generally, when a contrast value of an object is lower, it is difficult to detect a focus of an optical lens system by a phase differential type focus detector. In a conventional phase differential type focus detector, output signals from a photoelectric transfer element array are amplified with respect to a dark output voltage thereof (hereinafter, this amplifying mode is abbreviated as NM mode). Alternatively, output signals of a photoelectric transfer element array are amplified with respect to an average voltage of the output signals (hereinafter, this amplifying mode is abbreviated as LC mode). The LC mode is effective when the contrast value of the object is lower but the luminance thereof is higher. In the optical equipment having only one photoelectric transfer element array such as an AF-SLR camera in the initial stage, one of the above-mentioned NM mode and LC mode is selected for amplifying the output signals of the photoelectric transfer element array corresponding to level of the luminance of the object.
In recent years, it is proposed that a plurality of photoelectric transfer element arrays are respectively provided in different focusing areas so as to focusing an optical lens system on an object, for example, positioned distant from the center of a frame of a viewfinder of the camera. In such an equipment, there is a possibility that a first object having lower contrast value and a second object having a higher contrast value are respectively positioned on different focusing areas. Hereupon, when all the output signals of the photoelectric transfer element arrays are amplified by the above-mentioned NM mode, voltage of amplified signals with respect to the object having higher contrast value becomes too high to be treated in a CPU (Central Processing Unit). Alternatively, when all the output signals of the photoelectric transfer element arrays are amplified by the above-mentioned LC mode, variation of amplified signals with respect to the object having lower contrast value is too small detect the focus of the optical lens system.
In the phase differential type focus detector, each photoelectric transfer element array has a standard portion and a reference portion respectively configured by a plurality of pixels. The output signals are series of voltage signals from the pixels. There are many cases to call the output signals as data of pixels in the following explanation. The output signals of the standard portion are compared with the output signals of the of the reference portion for detecting a position where a pattern of the output signals of the standard portion is coincide with a pattern of the output signals of the reference portion. That is, the position shows the focus of the optical lens system.
In an equipment using the phase differential type focus detector, a plurality of objects disposed at different positions or a plurality of different portions of the same object having substantially the same pattern of contrast are rarely focused on different portions of the same photoelectric transfer element array. When data of pixels of the standard portion corresponding to a predetermined object or a predetermined portion of the object are compared with data of pixels of the reference portion corresponding to a different object or a different portion of the object having substantially the same pattern of contrast, the data of pixels of the standard portion is misjudged to coincide with the data of pixels of the reference portion. For preventing such misjudgment, the data of pixels of the standard portion are divided into a plurality of blocks in a manner so that a part of each block overlaps another block. The data of pixels in each block of the standard portion are compared with all the data of pixels of the reference portion.
A configuration of the photoelectric transfer element array having the standard portion and the reference portion and a method for comparing the data of pixels in the conventional phase differential type focus detector is described with reference to FIG. 17. The standard portion 51 and the reference portion 52 of the photoelectric transfer element array are actually disposed on the same line. However, the standard portion 51 and the reference portion 52 are disposed in parallel with each other in FIG. 17 so as to make the explanation of the comparison of the standard portion 51 with the reference portion 52 easy.
When the data of pixels of a block S3 disposed left hand in the standard portion 51 are compared with all the data of pixels of the reference portion 52, the data of pixels of the block S3 is shifted one by one from the left hand to the right hand in the figure for correlation operation. As can be seen from FIG. 17, the number of pixels of the reference portion 52 is larger than that of the standard portion 51.
The data of pixel of the block S3 are designated by b(j) (j=1 to N). The data of pixels of the reference portion 52 are designated by r(j) (j=1 to T). A number of pixels which are to be shifted (order of correlation operation) is designated by k (k=0 to (T-N)). A discordance quantity Hn(k) shown by the following equation can be used as a value for showing a ##EQU1## degree of coincidence of the correlation operation. Generally, the higher the decree of coincidence of the correlation operation is, the smaller the discordance quantity Hn(k) is.
The phase differential type focus detector, however, is incorporated in a highly integrated equipment such as an AF-SLR camera. Since size of the focus detector is restricted, the number of pixels of the photoelectric transfer element array (including the standard portion and the reference portion) can not be made so large. When the data of pixels of the block S3 disposed in the vicinity of left end of the standard portion 51 are compared with all the data of pixels of the reference portion 52, the reference portion 52 has sufficient number of pixels to be compared in the right hand in the figure, but it has a few pixels to be compared in the left hand in the figure. Similarly, when data of pixels of a block (not shown in the figure) disposed in the vicinity of the right end of the standard portion 51 are compared with all the data of pixels of the reference portion 52, the reference portion 52 has sufficient number of pixels to be compared in the left hand in the figure, but it has a few pixels to be compared in the right hand in the figure.
Hereupon, there is a possibility that the data of pixels in the vicinity of right end of the block S3 in the standard portion 51 coincide with the data of pixels in the vicinity of the left end of the reference portion 52. The conventional focus detector, however, cannot be detected the focus of the optical lens system due to the impossibility of the data comparing in spite of the possibility of the focus detection.
Furthermore, in the equipment having a plurality of photoelectric transfer element arrays, it is a problem to select a defocus with respect to the focus detection which is to be used in an operation of the lens driving among a plurality of defocuses obtained from the photoelectric transfer element arrays. A nearest position priority method compares two defocuses and selects the larger one. That is, the nearest position priority method prefers the object positioned near to the equipment. A reliability priority method calculates two contrast values from the data of pixels of the photoelectric transfer element arrays disposed in the lateral direction and the longitudinal direction and selects a defocus of the photoelectric transfer element array showing the higher contrast value. A lateral direction priority method selects the defocus of the photoelectric transfer element arrays disposed in the lateral direction. In the equipment simply adopting the reliability priority method or the lateral direction priority method, when the objects are respectively disposed at positions near to and far from the equipment, the focus of the optical lens system cannot be focused on the desired object.
Generally, performance for focus detection of the focus detector in the lateral direction is not necessarily the same as that in the longitudinal direction due to not only errors in an assembly of the focus detector but also errors in working of the parts. Furthermore, the sensitivity of the photoelectric transfer element arrays are not the same. Especially, when a photoelectric transfer element array is obliquely disposed with respect to an optical axis of an optical lens system, an error with directional dependency occurs with respect to an object disposed in the oblique direction against the photoelectric transfer element array. If the position or direction of the focus detector is adjusted in order to reduce the error of the focus detection in one of the lateral direction and the longitudinal direction, the error of the focus detection in the other direction which is not adjusted would remain or be increased.
In an equipment adopting the nearest position priority method, when the difference of two defocuses obtained from the photoelectric transfer element arrays disposed in the lateral direction and the longitudinal direction are small, far and near of the positions of the objects will be reversed due to the above-mentioned error with directional dependency under certain circumference. Similarly, in an equipment adopting the reliability priority method, a pair of photoelectric transfer element arrays having a larger error cannot be function.